Apparatus for preventing casing damage due to formation compaction

ABSTRACT

A method and apparatus for preventing casing damage due to high axial loading in a well as a result of compaction of the surrounding earth formations. The method consists of providing specially sized and located grooves on both the inside and outside diameters of a section of well casing. The grooves are designed and spacers may be used in some grooves to ensure symmetrical deformation of the casing when subjected to a compressive loading.

BACKGROUND OF THE INVENTION

The present invention relates to a special section of well casing thatis particularly useful in offshore wells that have under-compactedformations. As hydrocarbon production has moved into deeper watersoff-shore, the producing formations have often been younger,under-compacted reservoirs. As these reservoirs are produced, theycompact which results in axial loading and casing deformation. In manycases, the deformation of the casing has been extensive enough to causeloss of the producing wells. Since offshore production is typically froma structure having limited space, the loss of a well can seriouslyaffect the total production from the structure. Well replacement istypically difficult, if at all possible, and always results insignificant additional expense. When conditions preclude wellreplacement, the hydrocarbon reserves are lost forever.

One obvious solution to the problem of casing damage caused by axialload due to formation compaction would be to provide some sort of slipjoint in the casing that would allow the casing to shorten withoutdamage such as buckling or collapsing. While the slip joint would solvethe problem of buckled or collapsed casing, it requires seals or othermeans for isolating the formation from the interior of the casing.

In addition, if slip joints are used as part of a well casing, somemeans must be provided for preventing the slip joint from collapsing asthe casing is installed in the well. In addition to preventing the slipjoint from collapsing, structure must be included for preventing it fromextending beyond its limits as the casing is hung free in the well. Bothof these requirements involve complicated mechanical arrangements thatare, of course, subject to failure in a well. Obviously, if the slipjoint compresses during installation of the casing, it will not provideprotection against casing collapse when the reservoir compacts.

SUMMARY OF THE INVENTION

The present invention solves the problem of casing damage due to axialload from formation compaction by providing a special casing joint whichcan compress or shorten as the formation compacts. The special casingsection is also provided with means to limit its elongation so that itwill not be unduly extended as the casing is installed in the well.While the elongation of the special casing section is limited, it isprovided with unlimited compression or shortening ability within itsdesigned limits. The shortening of each casing section is limited butpractically any desired shortening can be obtained by using multiplelengths of the special casing section. In particular, the inventionprovides for a finite shortening per unit length of the special casingsection. Thus, when additional provision must be made for shortening ofthe casing section as a result of compaction of the reservoir, one mayprovide additional lengths of the special casing section. In particular,the invention allows axial shortening of the casing string withoutunacceptable radial deformation and without requiring the seals thatwould be necessary in the case of a slip joint.

The special casing section can be visualized as a bellows or accordianthat permits shortening of its overall length in response to axial load.It can best be described as a piece of casing or thick-walled tubularproduct that has grooves machined on both its outside and insidediameters. The grooves are of a particular shape and are located atprecise axial distances from each other. Further, the number of grooveson the outside diameter is the same as the grooves on the innerdiameter. The grooves typically have a U-shaped cross section in whichthe depth and axial location of the U's as well as their depth inrelation to the thickness of the casing wall is closely controlled. Inparticular, the depth of the grooves is controlled so that an overlapoccurs between the grooves formed on the inner diameter and those formedon the outer diameter. These dimensions are controlled so that when thespecial casing section is subjected to an axial load as occurs when theformation surrounding the casing compacts, the U-shaped grooves willclose. The special casing section is designed so that the ends of thegrooves adjacent the inner or outer surface of the casing close andconvert the U-shaped groove into a teardrop-shaped groove.

The dimensions of the grooves are also controlled in relation to thematerial from which the special casing section is formed to ensure thatthe grooves, when collapsed, will form a teardrop section. This isnecessary to ensure that the special casing section does not buckle orotherwise deform because the grooves failed to close. In some cases, aprotective sleeve is placed around the special casing section and isdesigned to provide the strength required to prevent lateral buckling ofthe special casing section when the axial load is applied. In addition,the outer sleeve also protects the grooves and keeps them clear ofcement or formation material as the special casing section is installedin the well. Obviously, if the grooves on the outer diameter becameclogged with cement or formation material, it could interfere with thefunctioning of the tool. The protective sleeve also limits axialextension due to high axial tension load when the casing is beinginstalled in the well.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be more easily understood from the followingdetailed description of a preferred embodiment when taken in conjunctionwith the attached drawings in which:

FIG. 1 is an elevation view of the complete special casing section.

FIG. 2 is an elevation view of the special casing section shown in FIG.1 with the outer protective sleeves removed.

FIG. 3 is an enlarged cross section of the grooves shown in the specialcasing section of FIG. 2.

FIG. 4 is a cross section of the grooves shown in FIG. 3 shown in acollapsed configuration.

FIG. 5 is a cross section of one portion of the casing shown in FIG. 1with the protective sleeve installed.

FIG. 6 is a modified form of the casing shown in FIG. 1 shown half inelevation and half in section.

DESCRIPTION OF THE PREFERRED EMBODIMENT

In FIG. 1 there is shown an elevation view of the special well casingassembly 10. The special casing assembly is provided with tubularconnections 11 and 12 at each end that can be used to join the specialcasing to the conventional well casing or to additional special casingassemblies. In the unit shown in FIG. 1, two separate sections ofgrooves are provided with the grooves being covered by protectivesleeves 13 and 14.

In FIG. 2, there is shown the details of the casing section of theassembly shown in FIG. 1. Casing section 20 is provided with a series ofgrooves 23 on its outer surface at two separate locations 21 and 22.These separate sections of grooves are used on the casing 20 to providemaximum amount of axial shortening in a given length of the casing 20.Obviously, the amount that the casing 20 can be shortened by compressionwill depend upon the number and size of grooves provided. It isnecessary to provide two separate sections of grooves because only alimited number of grooves can be formed on the interior diameter of thecasing 20. The number of interior grooves is limited by difficulty ofmachining the interior grooves at a great depth within the casing. It isobvious that if a larger diameter casing were used, the forming of thegrooves on the interior diameter of the casing would be simplified andin some instances, it may be possible to combine the two sections ofgrooves, 21 and 22, in a single group of grooves or provide a continuousseries of grooves on the interior of casing 20.

Referring to FIG. 3, there is shown an enlarged cross section of some ofthe exterior and interior grooves formed on the casing section 20 ofFIG. 2. In particular, the exterior grooves 23 formed on the outersurface are equally spaced between the interior grooves 24 and when theuppermost groove is formed on the inner surface, then it is desirablethat the lowermost groove be formed on the outer surface as shown inFIG. 3. The uppermost and lowermost grooves maybe located either onouter or inner surfaces and regardless of their positions, the uppermostand lowermost grooves will collapse only approximately one-half thedistance of the remaining grooves. It should be noted that while 14grooves are shown on the outer surface of the casing 20 of FIG. 2, only6 are shown in FIG. 3 due to the enlarged scale of the drawing of FIG.3.

Obviously, grooves of different dimensions and different spacings may beused with the width and number of the grooves depending upon the totaldesired shortening of the casing section. The geometry of the groovesand the material used in the casing control the load required to shortenthe casing. In the case of 71/4" well casing having a wall thickness of11/16ths of an inch and formed of 85,000 psi minimum specified yieldstrength steel, grooves having the following dimensions provideexcellent results when the casing is subjected to axial load. The widthA of the grooves is 0.250 inches while the groove has a depth whichleaves a wall thickness B at the bottom of 0.188 inches and the spacingC between the center line of grooves on the inner surface and those onthe outer surface is 0.500 inches. Using these dimensions and providinga near perfect semicircle at the bottom of the groove as shown in FIG.3, the grooves will have a radial overlap between the maximum depth ofthe inner groove and those of the outer groove of approximately 0.313inches. While grooves having parallel sides are preferred for ease ofmanufacturing, V-shaped grooves having a circular bottom could also beused. Further, the uppermost and lowermost grooves can both be locatedon either the outer or inner diameters in place of an equal number ofgrooves on the inner and outer diameters as shown.

The foregoing dimensions can be varied to vary some of thecharacteristics of special casing section. For example, to increase thecollapse strength one can increase the spacing "C" between the grooves.As an alternative, the wall thickness B could be increased to increasethe collapse strength of the casing.

Referring to FIG. 4, there is shown a section of the casing of FIG. 3 ina fully collapsed or shortened condition. As shown, the uppermost groove25 on the inner surface is provided with a spacer 27 while the lowermostgroove 28 on the outer surface is provided with a similar spacer 26. Thespacers 27 and 26 have a thickness equal to one-half of the groove widthto limit the closure of the upper and lowermost grooves to one-half thewidth of the groove. While it is preferable to use spacers 27 and 26 inthe end grooves, tests have shown that the spacers can be eliminated andthe casing will collapse uniformly under an axial load. In contrast, theremainder of the grooves can completely close until the outer edges ofthe groove touch to form a teardrop-shaped closed section 30 as shown inFIG. 4 with slight bulges forming on the inner and outer surfaces asshown.

Referring to FIG. 5, there is shown a cross section of the specialcasing shown in FIG. 1 with the protective sleeve 13 installed. Only theupper portion of the special casing shown in FIG. 5 is shown and thelower portion containing the protective sleeve 14 in the second set ofgrooves is omitted in the illustration. The protective sleeve 13 isprovided with a vent hole 40 near its upper edge and a vent hole 41 nearits lower edge. The vent holes serve to equalize the pressure betweenthe interior of the sleeve and the exterior as the special casingsection is installed in the well. In addition, the holes can be used forfilling the void between the casing and protective sleeve with oil orgrease. If the void is filled, the lower vent hole 41 should be closedwith a threaded plug and the upper vent hole 40 sealed with a deformableplug that will move to allow the pressure to equalize. O-ring seals 42and 43 are provided at the upper and the lower ends of the protectivesleeve to exclude borehole debris from the interior of the sleeve and toallow pumping the cavity full of oil or grease.

The protective sleeve is secured to the casing section 20 by grooved andthreaded rings at its upper and lower ends respectively. The upperconnection is formed by annular grooves 50 on the interior surface ofthe sleeve and a C-ring shaped grooved section 52 that is secured to thecasing section 20. The threaded connection at the lower end is formed bythreads 51 on the inner surface of the sleeve and the threaded C-ring53. It should be noted that the grooves and threads are modifiedbuttress-type with the inclined surfaces of the buttress facing eachother as shown in FIG. 5. The lower threaded C-ring is held in positionand prevented from turning by a set screw 54 which passes through a slotbetween the end of the ring and threads into the wall of the casingsection 20 while the upper grooved C-ring 53 is free to rotate. Thisallows the lower ring to be threaded onto the outer sleeve and thesleeve to slide upwardly as explained below.

The sleeve is assembled on the exterior of the casing 20 by firstinstalling the upper C-ring 52 on the exterior surface of the casing 20.The sleeve can then be slid over the bottom of the casing section 20. Asthe sleeve is moved upward the C-ring 52 will slide up until it isaligned with recess 57 at which point it will be forced into the recessby the sleeve and the sleeve will ratchet over the ring. The sleeve israised further until the lower end of the sleeve clears the recess 56.The C-ring 53 can then be installed in the recess 56 and the ends of theC-ring positioned so that the set screw 54 can be installed. The sleevecan then be lowered along the casing 20 by sliding the sleeve over theC-ring 53 until both C-rings are aligned with grooves 50 and threads 51.The sleeve can then be rotated relative to the body. As the sleeve isthreaded onto the C-ring 53, the C-ring 52 will contact shoulder 58 andC-ring 53 will contact shoulder 59 as shown in FIG. 5.

While the casing cannot stretch as it is installed, the compression orshortening of the casing section is limited only by the number ofgrooves 23 and 24 and their width. As the casing section 20 collapses orshortens by the closing of the grooves 23 and 24, the lower C-ring 53will slide down the outer surface of the casing section 20. Once theC-ring 53 aligns with the recess section 56, the sleeve will slide overthe threads on the C-ring 53 and allow the casing to be shortened untilthe grooves completely close as shown in FIG. 4. The similar actionoccurs at the top C-ring.

FIG. 6 illustrates a modified form of the special casing section that iseasier to manufacture while providing a means for obtaining any desiredaxial shortening in a single casing section. The modified casing has twoend sections 63 and 64 that are joined to a center section 65 by welds70 and 71. Obviously, as many center sections as needed can be used toprovide any desired axial shortening. The center section may be providedwith annular flanges 66 having rounded corners. The flanges 66 stabilizethe special casing and help ensure that the grooves will close and thecasing shorten under an axial load without buckling. The flanges 66slide on the inner surface of the modified protective sleeve 60. Themodified sleeve 60 is the same as sleeve 13 except it is longer toaccommodate the increased length of the special casing. The sleeve 60 isattached to the special casing using the same grooved ring 52 andthreaded ring 53 as described above and illustrated in FIG. 5.

The modified casing is assembled in the same manner as described aboveand illustrated in FIG. 5. It should be noted that the end sections 63and 64 are provided with larger outer diameter sections at their ends.This provides the wall thickness required for the ring and threadarrangement used to secure the protective sleeve to the casing section.The use of larger diameter sections at the ends of sections 63 and 64allows the use of thinner walled sections where the grooves are formed.This configuration facilitates the manufacture of the sleeve by allowingthe use of a constant diameter for the sleeve and still provideclearance between the sleeve and the casing section. This also producesstiff end sections and optimum grooved sections and uniform axialshortening of the casing section under axial load.

From the above description it can be readily appreciated that thisinvention has provided a special casing section which can shorten adesigned amount thus minimizing the axial load and prevent buckling orcollapsing. In addition, provisions are made for limiting the elongationof the special casing section so that the casing can be installed as aportion of the regular well casing and hung in a well without danger ofthe casing elongating to an extent that would cause parting of thecasing. The use of the protective sleeve on the outer surface of thecasing serves the dual purpose of providing strength to the casing toprevent its lateral buckling or otherwise deforming and to excludeformation debris from the grooves formed on the outer surface of thespecial casing. The protective sleeve also carries a portion of theaxial load when the casing is being installed in a well and hanging freein the well. The grooves formed on the interior surface of the casingcan be filled with a deformable plastic to exclude debris from thesegrooves. The deformable plastic will be extruded from the grooves whenthe casing is subject to a compression load.

The protective sleeve can be eliminated when the weight of the casingsuspended below the special casing does not exceed its design limits andthe special casing is laterally supported by the cement.

What is claimed is:
 1. A special well casing section, said casingsection comprising:(a) a section of tubular well casing and (b) a meansallowing axial loading of said well casing without buckling said wellcasing, said means comprising (i) a plurality of first annular grooves,said first grooves being formed on the outer surface of said wellcasing, said first grooves being equally spaced; (ii) a plurality ofsecond annular grooves, said second grooves being formed on the innersurface of said well casing, said second grooves being equally spacedbetween said first grooves; and (iii) said first and second grooveshaving substantially identical U-shaped cross sections, the depth ofsaid first and second grooves being sufficient to allow the bottom ofsaid grooves to overlap and the axial spacing between adjacent first andsecond grooves being substantially equal to substantially twice thewidth of said grooves and the width of the grooves being such that asaxial loading occurs, ends of the U-shaped grooves substantially closeand convert the U-shaped grooves into teardrop-shaped grooves.
 2. Thecasing section of claim 1 and in addition, a protective sleeve, saidsleeve surrounding at least the portion of the outer surface of saidwell casing containing said first grooves, said protective sleeve beingattached to said well casing in a manner to permit limited elongationand shortening of the well casing to the extent required to allow saidfirst and second grooves to close until the outer edges of each grooveare in contact.
 3. The casing section of claim 2 wherein the protectivesleeve is attached to the well casing by a system comprising:a series ofthreads formed on the inner surface of the protective sleeve at one endand a third series of annular grooves formed on the inner surface of theprotective sleeve at the other end; a series of threads positioned onone end of said well casing beyond the location of said first and secondannular grooves and a fourth series of annular grooves positioned on theother end of said well casing beyond said first and second annulargrooves, said fourth series of annular grooves being constructed andpositioned on said well casing to permit said protective sleeve to beinstalled on said well casing by sliding the end of the protectivesleeve containing said third series of annular grooves over said threadson said well casing and over the section of the well casing containingsaid first and second series of annular grooves until the threads onsaid protective sleeve contact the threads on the well casing and thenthreading the protective sleeve onto the well casing.
 4. The casing ofclaim 3 wherein the threads on the well casing comprise a separate "C"shaped ring member positioned in a recess formed on the outer surface ofthe well casing.
 5. The casing of claim 4 wherein the fourth series ofannular grooves on the well casing comprise a second separate "C" shapedring member positioned in a recess formed on the outer surface of thewell casing.
 6. The casing of claim 5 wherein the axial length of therecesses allows said "C" shaped rings to be forced into said recesses.7. The casing of claim 6 wherein all of said threads and third series ofannular grooves on said protective sleeve and the threads and annulargrooves on said first and second "C" shaped ring have a modifiedbuttress cross section with the buttress side of the thread and annulargrooves on each of said "C" shaped rings projecting in oppositedirections.
 8. The casing of claim 7 wherein the buttress side of thethread on said "C" shaped rings projects in a direction that permits atleast the third series of annular grooves on the protective sleeve tomove relative to the second C-ring when said second C-ring section ismoved to a position on the well casing having a diameter that is lessthan the diameter of the recess.